This application is based on application No. 2000-26266 filed in the Korean Industrial Property Office on May 17, 2000, the content of which is incorporated hereinto by reference.
(a) Field of the Invention
The present invention relates to a negative active material for a lithium secondary battery, and more particularly, to a negative active material for a lithium secondary battery, into which electrolyte is easily immersed.
(b) Description of the Related Art
Generally, carbonaceous material is used as a negative active material for a lithium secondary battery. Carbonaceous materials are classified into amorphous carbon and crystalline graphite, and crystalline graphite includes artificial graphite and natural graphite.
Natural graphite, or platelet-type graphite produced by graphitization then pulverization of coke, are candidates for positive active material in lithium secondary batteries as they are inexpensive and exhibit voltage flatness, high capacity and efficiency. But a low density and a high surface area of natural graphite and platelet type artificial graphite having plate-shaped or flake type particle structures requires a large amount of solvent during the preparation of active material slurry so that adhesion between slurry and a current collector is reduced and thus the cycle life characteristics deteriorates. The slurry including a large amount of solvent has poor fluidity so that a step of coating the slurry on the current collector is extremely slowly performed to obtain uniform thickness slurry coating layer on the current collector.
In addition, the plates of the natural graphite or platelet type artificial graphite active material lie parallel on a current collector as shown in FIG. 1 during the electrode preparation. In FIG. 1, A refers to the basal plane, B refers to the edge, 1 refers to the collector, and 2 refers to the plate shaped graphite. This result allows the graphite to be severely compressed to the collector, which results in the poor electrolyte-immersibility to the graphite active material and the decreased high-rate cycle life characteristics. Namely, the basal planes of graphite A are severely compressed to the collector 1 so that lithium ions have difficultly intercalating into the graphite, thereby decreasing capacity. Due to such a severe compression, the adhesion between graphite and the collector is reduced and thus the cycle life characteristics deteriorate.
Such a severe compression is detrimental in a lithium polymer battery. Generally, positive active material slurry includes plasticizer used for making perforations in the electrode. That is, the plasticizer is extracted by using an organic solvent and forms a plurality of micro-spaces in the electrode. These micro-spaces increase the contact area between the active material and the electrolyte. However, the severe compression renders it is difficult for extraction solvents, such as ether or methanol, to reach the plasticizer so that the plasticizer can be completely extracted from the electrode, thus the electrochemical characteristics deteriorate.
It is an object of the present invention to provide a negative active material for a lithium secondary battery which exhibits good high-rate charge and discharge efficiency.
It is another object to provide the negative active material for a lithium secondary battery into which electrolyte is easily immersed.
In order to achieve these objects and others, the present invention provides a negative active material for a lithium secondary battery including graphitized coke, and non-flat artificial graphite. The graphitized coke includes a graphitization catalyst.